The difference is magnitude, the benefits that European countries provide has made them a very attractive destination for immigration, so it stands to reason that something like UBI would make them even more so. No matter which side of that debate you're on theres a point where the math breaks down and some difficult choices need to be made, either you provide these generous benefits to your citizens or you have a generous immigration policy, but both of them together may prove unsustainable

Which is why unchecked immigration is already a problem. Increasing the benefits without dealing with unchecked immigration is going to make things worse.